A stress sensor has been disclosed in Japanese Unexamined Patent Application Publication No. 2000-267803, which is capable of grasping the direction and magnitude of a stress applied to a post bonded to or integrated with a surface of an insulating board from variation in resistance of a plurality of resistor elements caused by stimulation provided thereto resulting form the application of the stress to the post. The formation of the resistor elements thus disclosed, which elements form a strain gage, is performed by screen printing of all constituent elements of the resistor elements on a surface of a ceramic board.
The structure is formed as shown in FIG. 15, in which four resistor elements 22 are disposed on two lines, which are along a surface of an insulating board 20 and perpendicularly intersect each other at the center of a surface the insulating board 20, and are provided at substantially the same distance from the intersecting point. In addition, in the structure described above, a post 30 having a square bottom surface is bonded so that the center thereof coincides with the center of the surface of the insulating board 20 and that individual sides of the bottom surface of the post 30 oppose the respective resistor elements 22. In addition, board terminal parts 24 are provided at end portions along the entire periphery of the insulating board 20 at approximately regular intervals. Since conductors (electrodes) to be connected to the resistor elements 22 and the board terminal parts 24 are formed on the surface of the insulating board 20 by a screen printing method, they have a uniform (predetermined) height from the surface of the insulating board 20.
In recent years, in addition to stress sensors having the structure in which all constituents for constituting resistor elements are formed by screen printing on a surface of a ceramic board, development of stress sensors using an insulating board provided with conductors, which are obtained by partly removing a conductor layer on a surface as remains, has also been carried out. The reasons for this is that, compared to thick film techniques such as a screen printing technique, the conductors of the insulating board described above can be easily processed to have fine patterns, and in addition, an advantage of low manufacturing cost can also be obtained.
However, when the insulating board for a stress sensor is an insulating board having conductors which are obtained by partly removing a conductive layer on a surface as remains, and when the stress sensor uses parts of the conductors 9 as electrodes and resistor elements as a strain gage, each element formed of a resistor provided between a respective pair of the electrodes on the surface of the insulating board, a problem different from that of the conventional technique described above may arise.
In the past, the electrodes (conductors) constituting the resistor elements were formed by a screen printing method, and in the above technique, the conductors are formed of the remains obtained by partly removing the conductor layer on the surface; hence, the problem is generated by the difference described above.
The difference described above is schematically shown in FIG. 7. FIG. 7(a) is a schematic cross-sectional view of a resistor element 8 using conductors (circuit pattern electrodes 1) as electrodes, which are obtained by partly removing a conductor layer on a surface of an insulating board 3. FIG. 7(b) is a schematic cross-sectional view of a resistor element 8 using conductors (resistor-element electrodes (hereinafter referred to as “thick electrodes)) obtained by screen printing, which is one of thick film techniques.
The conductor height shown in FIG. 7(a) mostly depends on the thickness of the conductor layer which is originally disposed on a surface of the insulating board 3 and is formed of copper or the like. In general, this thickness is approximately from 18 to 36 μm. In addition, when the insulating board 3 is a so-called double-sided board, in which the conductors 9 on both surfaces of the insulating board 3 are connected to each other through a conductive material formed inside a through-hole by plating, the conductive material may be further adhered to the conductors 9 by this plating, and as a result, the height thereof may be further increased to approximately 40 to 70 μm in some cases. On the other hand, the thickness of a thick film electrode 13 shown in FIG. 7(b) can be determined optionally to some extent, and is generally set to approximately 10 μm.
Next, the difference in cross-sectional shape between the circuit pattern electrode 1 and the thick film electrode 13 will be described. The circuit pattern electrode 1 has a cross-sectional shape similar to a rectangular shape, and it is understood that the circuit pattern electrode 1 has surfaces approximately perpendicular to that of the insulating board 3 (FIG. 7(a)). On the other hand, the thick film electrode 13 has a curved cross-sectional shape primarily formed of components inclined with respect to the surface of the insulating board 3, and it is understood that the thick film electrode 13 is primarily composed of surfaces smooth with respect to the surface of the insulating board 3 (FIG. 7(b)).
Due to the difference between the circuit pattern electrode 1 and the thick film electrode 13, compared to the resistor element 8 (FIG. 7(b)) using the thick film electrodes 13 as the electrodes, the resistor element 8 (FIG. 7(a)) using the circuit pattern electrodes 1 as the electrodes has a large variation in resistance. The reason for this is that it is difficult for the former to form a resistor 2 having a uniform shape. When the variation in resistance is large, in a so-called trimming step in which adjustment is performed to obtain a desired resistance, a resistor element 8 in which an excessively long trimming groove must be formed and a resistor element 8 in which a trimming groove is not substantially necessary are both present at the same time. Although the resistances are equal to each other, when the trimming lengths are extremely different from each other as described above, due to the change in ambient environment, particularly an ambient temperature, the resistance stability cannot be obtained. That is, even when the nominal resistances are the same, in this case, resistor elements 8, in which the variation in various properties other than the resistance is large, are formed. In addition, in a stress sensor in which resistor elements 8 having trimming grooves are used as a strain gage, minute cracks around the trimming grooves will grow by the use for a long period of time, and as a result, the original resistance may not be maintained in some cases.
As described above, compared to the case in which the thick film electrodes 13 are used, when the circuit pattern electrodes 1 described above are used, the shape of the resistor 2 having a large thickness formed between the electrodes by a thick film technique such as a screen printing technique becomes unstable, and it has been, believed that there are two reasons for this problem.
The first reason is that the height of the circuit pattern electrode 1 is large as described above. For example, in the case in which a film for the resistor 2 is formed by a screen printing method, an approximately predetermined amount of a resistor paste which passed through a mask (screen) is supplied between a pair of the circuit pattern electrodes 1. Depending on various factors such as an ambient temperature, a paste temperature, and a holding time for fixing the shape of the resistor 2 obtained by firing or curing performed after screen printing, the shape of the fixed resistor 2 varies. For example, due to a high ambient temperature or the like, when the paste viscosity is low, the upper surface of the resistor 2 between a pair of the circuit pattern electrodes 1 becomes approximately flat, and as a result, a relatively stable shape is obtained. On the other hand, when a paste having a high viscosity is supplied between a pair of the circuit pattern electrodes 1, the paste is solidified by firing/curing while maintaining the original shape, which is formed when the paste is supplied, to some extent. It has been believed that this phenomenon becomes apparent when the resistor paste contains a thermosetting resin. The reason for this is believed that decrease in paste viscosity is not likely to occur even when the paste is heated. When the height of the circuit pattern electrode 1 is large, an area around the circuit pattern electrode 1 becomes a paste easy-flow region when the viscosity of the resistor paste is high. The reason for this is that the paste in the vicinity of the peak of the circuit pattern electrode 1 moves from a higher position to a lower position by its own weight.
In addition, in the case in which the film for the resistor 2 is formed by a screen printing method and in which the height of the circuit pattern electrode 1 is excessively large, when the resistor paste is allowed to pass through a mask by a squeegee, the squeegee is likely to collide against the circuit pattern electrode 1. Hence, the squeegee supplies the resistor paste through the mask in an irregular manner, resulting in the variation in amount of the resistor paste supplied through the mask and, in addition, in the deviation of the position at which the resistor paste is supplied. Accordingly, the phenomenon in which the shape of the resistor 2 formed between the circuit pattern electrodes 1 is unlikely to be stable becomes more serious.
The second reason is that the circuit pattern electrode 1 has a surface approximately perpendicular to the surface of the insulating board 3. It has been very difficult to control the resistor 2 present on the approximately perpendicular surface to have a predetermined thickness. The reason for this is that, as described above, when the paste in the vicinity of the peak of the circuit pattern electrode 1 moves from a higher position to a lower position by its own weight, it is difficult to estimate how the paste moves along the approximately perpendicular surface. In addition to the presence of the approximately perpendicular surface, the second reason described above makes the shape of the resistor 2 unstable in combination with the first reason. That is, when the height of the circuit pattern electrode 1 is small, the distance is small along which the paste in the vicinity of the peak of the circuit pattern electrode 1 described above moves by its own weight from a higher position to a lower position, and as a result, the variation in resistance caused by the difference of the thickness of the resistor 2 on the approximately perpendicular surface from that in the direction perpendicular thereto is small enough to be ignored.
This second reason is not only applied to the film formation of the resistor 2 by a thick film technique such as screen printing but is also applied to that of the film of the resistor 2 for the resistor element 8 by a thin film technique such as sputtering. The reason for this is that, for example, when sputtering is performed for the circuit pattern electrode 1 having a large height and an approximately perpendicular surface, it is difficult to control the thickness of the resistor 2 adhered to this perpendicular surface to be a predetermined value. That is, even in the film formation of the resistor 2 by a thin film technique, it is difficult to control the shape of the resistor 2 to be uniform, and as a result, the variation in resistance is liable to occur.
Accordingly, an object of the present invention is to decrease the variation in resistance of a resistor element having a resistor film formed between a pair of electrodes on a surface of the insulating board 3, the electrodes being parts of conductors obtained as the remains by partly removing a conductor layer on the surface of the insulating board. In addition, the present invention provides a stress sensor using the resistor elements described above.